1. Field of the Invention
The present invention relates to a method of canceling a short circuit in a wire cut electric discharge machine, which cancels the short circuit generated between the wire and a workpiece during the electric discharge machining, and in particular, to a method capable of canceling a short circuit generated in the perpendicular direction to the wire advancing direction.
2. Description of the Related Art
The wire cut electric discharge machine generates a pulse discharge in a space between the wire and workpiece. Part of the workpiece is thereby melted. Then, the melted part is blown off by means of the machining liquid from a nozzle, and this operation is continuously carried out. Thus the workpiece is formed into a desired shape. However, during the wire cut electric discharge machining, if a short circuit is generated in a space between the wire and workpiece, the pulse is not discharged. For this reason, the machining is interrupted.
FIG. 9 is a view showing a state between the wire and workpiece during the discharge machining; FIG. (A) shows a normal state, and FIG. (B) shows a short-circuited state. As illustrated in FIG. 9 (A), the wire 20 is supported by means of the upper and lower guides 7 and 10, and a space with a very short distance .DELTA. is kept between the wire 20 and workpiece 6 by the discharge. Moreover, repulsive force by the discharge machining is generated in the reverse direction relative to the advancing direction 21 of the wire 20. Therefore, deflection of the wire 20 occurs in the center portion 20A of the workpiece thickness. Thus, the discharge machining is carried out under such a state. On the contrary, as illustrated in FIG. 9 (B), once the workpiece 6 and the wire 20 come into contact with each other and a short circuit occurs there, the repulsive force by the discharge machining decreases, while the deflection is gone out of the wire. For this reason, the contact state of the workpiece 6 and the wire 20 is maintained, so that the machining can not be continued. In view of this point, if such a short circuit is detected, the wire is once reversed along the machined path therefrom, and if the cancellation of the short circuit is detected during the reversing operation of the wire, the wire is again returned to the initial short-circuited spot, thus the machining is restarted. Such a method of canceling the short circuit mentioned above is very effective in a short circuit generated in the wire advancing direction.
However, the short circuit is not necessarily generated in the wire advancing direction; for example, in the finish machining, the short circuit may occur in the perpendicular direction to the wire advancing one. In this case, the method of canceling the short circuit mentioned above can not cancel the short circuit between the wire and the workpiece.
FIG. 10 is a view showing a state between the wire and workpiece in the finish machining. In the figure, the finishing allowance 6A of the workpiece 6 is ground by the discharge generated between the wire 20 and the workpiece 6, thus being subjected to the finish machining. In the case of such a finish machining, the offset direction 22 is ordinarily an open surface, so that repulsive force is applied in the oblique direction as shown by an arrow 23 to the advancing direction 21 of the wire 20. For this reason, the wire 20 receives repulsive force of not only the reverse direction 24 to the advancing direction 21 but also the perpendicular direction 25 to the same. As a consequence, deflection of the wire 20 occurs in the perpendicular direction 25. Accordingly, the short circuit is generated in not only the wire advancing direction 21 but also the reverse direction relative to the perpendicular direction.
The short circuit generated in the advancing direction is effectively canceled by the foregoing method of canceling a short circuit, in which the wire is reversed along the path previously machined, but it is impossible to cancel the short circuit generated in the perpendicular direction. In the case where the short circuit is not canceled, the information is given to the operator by an alarm. However, the operator is required to once interrupt the automatic operation pursuant to the alarm, and to move the wire by manual operation in order to cancel the short circuit. For this reason, there are problems that it is troublesome to cancel the short circuit, and impossible to perform perfectly unmanned operations during rough to finish machining.